1. Field of the Invention
This invention relates generally to telecommunications, and, more particularly, to a method and apparatus for adaptive DC level control for a telephone line card.
2. Description of the Related Art
In communications systems, particularly telephony, it is a common practice to transmit signals between a subscriber station and a central switching office via a two-wire bi-directional communication channel. A line card generally connects the subscriber station to the central switching office. A line card typically includes at least one subscriber line interface circuit (SLIC) as well as a subscriber line audio-processing circuit (SLAC). The primary functions of the line card may include supplying talk battery, performing impedance matching, determining whether telephonic equipment is on-hook or off-hook, and handling ringing signals, voice signals, and testing signals.
Subscriber line interface circuits generally include a switch-hook detection circuit that supervises telephone operation by detecting whether a telephone is either on-hook or off-book. An “off-hook” condition occurs when an end user lifts the handset of a telephone from the cradle, thereby activating the telephone's hook switch. Conversely, an “on-hook” condition occurs when the handset is placed back in the telephone cradle, thereby terminating the telephone service. Upon detecting the on-hook or off-hook condition, the switch-hook information is passed to the system software of the line card, which then either provides or terminates service.
Upon requesting service, a user may establish a connection with a remote user by dialing the telephone number of that user. Telephone dialing may be in the form of dial pulses or tones. Pulse dialing includes generating a series of electrical interrupts or pulses across the telephone line. One method of generating electrical pulses is by toggling between on-hook and off-hook states such that each transition from an on-hook to off-hook state represents one pulse. The number of pulses generated usually represents the digit that is dialed.
The off-hook/on-hook ratio (also referred to as make/break ratio) applied to the loop during pulse dialing is typically 40 to 60; that is, the loop is closed 40 percent of the time and is open 60 percent of the time. The break interval is generally allowed to vary from about 58 percent to 64 percent. However, because of the pulse distortion caused by the loop, the pulse receivers or detectors in the central office must be able to properly respond to a break interval of 55 percent to 65 percent. The dial rate can vary between 8 and 12 pulses per second.
Telephone systems need DC feed to control the battery feed to the subscriber loop. DC feed delivers enough power for long loop and gradually reduces the power for short loop. A digital signal processing algorithm controls the DC feed curve. During the transition from off-hook to on-hook, the loop impedance changes from low to high and the measured loop voltage between the tip and ring terminals goes from low to high. Generally, an electronic DC feed control is slow to react to sudden changes in the loop impedance. As such, if the loop impedance is high enough during an off-hook to on-hook transition, the DC feed driver may be saturated.
Saturation may occur because an impedance change on a subscriber loop causes a DC level shift between several volts to ten volts or more, depending upon the loop current. The SLAC devices are typically five-volt digital components with a maximum peak to peak operating voltage of less than five volts. Thus, a sudden DC level shift causes one or more components within the SLIC and SLAC devices to saturate, sometimes for as long as 80 milliseconds, which may interfere with normal operation of such devices.
During saturation, the measured loop voltage between the tip and ring terminals and the measured current may not accurately reflect the loop condition. Thus, an inaccurate measurement of the loop impedance may result in a false detection, thereby causing switch-hook distortion. In addition, switch-hook distortion may also occur during pulse dialing because of capacitance and inductance present on the subscriber line or may occur when additional telephones go off-hook/on-hook on the subscriber loop, thereby causing a sudden impedance fluctuation.
Particular care must be taken when implementing DC level control functionality in a line card that is capable of supporting voice and data. A line card capable of supporting both voice and data may employ an analog-to-digital converter (A/D), as well as a digital-to-analog (D/A) converter, for DC level control. The D/A converter may be provided with a stream of digitized input samples, which are then converted to an analog signal by the D/A converter. The range of values of the digitized samples will generally depend on the input signal that is digitized by the A/D converter. The D/A converter may introduce a substantial amount of quantization noise, depending on the sampling rate of the D/A converter and the architecture of the D/A converter. For example, with the zero order of D/A converter, the quantization noise is flat from 0 Hz to 128 KHz. With a first order D/A sigma-delta D/A converter, the noise peaks at half of the D/A sampling frequency, Fs. If the sampling frequency equals 256 HKz, then the noise peaks at 128 HKz, which is in the frequency bin for upstream data transfer. As a result, the noise introduced may interfere with the transfer of data in the line card, thereby increasing the bit error rate of data transfer.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.